At the conclusion of makkas barad (plague of hail) the Torah elaborates on precisely which plants were destroyed. Why is it important to know that the flax and barley were destroyed whereas the wheat survived? What lessons can be learned from which plants survived the barad?
According to the literal meaning of the pesukim in Shemos 9:31-32 the plants that were not damaged were those that did not stand firm as the hail fell. Those plants that were not yet developed were able to withstand the pressure of the hail. However, the fully developed, stronger plants which tried to stand up to the hail were decimated.
The message to Pharoh was clear. When you are confronted by Hashem whose power is so much greater than yours, how do you react? Do you attempt to stand firm and obstinate like the flax and barley. Thereby guaranteeing your destruction? Or, like the wheat, do you realize your limitations and refrain from attempting to battle the hail? Pharoh failed to see the obvious message in the makka and continued to be obstinate thereby sealing his own fate.
The Ohr Hachaim has a different suggestion regarding the significance of the wheat being spared. The medrash explains that this was miraculous, but Pharoh saw it as a sign of Hashem's weakness and limited ability. Rather than drawing the correct conclusion that Hashem decided exactly what to destroy and what to retain in a miraculous manner, Pharoh saw it as a justification to continue to question whether the makkos were merely magic.
The wheat survived the hail in order to teach Pharoh to repent. Pharoh drew the incorrect conclusion and continued along his evil path. Let us hear the messages that Hashem sends through events that occur to us and learn the lessons He is trying to teach.